Point-to-multipoint digital radio frequency transport

ABSTRACT

One embodiment is directed to a system for use with a coverage area in which one or more wireless units wirelessly transmit using a wireless radio frequency spectrum. The system comprises a first unit, and a plurality of second units communicatively coupled to the first unit using at least one communication medium. Each of the plurality of second units generates respective digital RF samples indicative of a respective analog wireless signal received at that second unit. Each of the plurality of second units communicates the respective digital RF samples generated by that second unit to the first unit using the at least one communication medium. The first unit digitally sums corresponding digital RF samples received from the plurality of second units to produce summed digital RF samples. The system is configured so that an input used for base station processing is derived from the resulting summed digital RF samples.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/662,948, filed on Oct. 29, 2012 entitled “POINT-TO-MULTIPOINT DIGITALRADIO FREQUENCY TRANSPORT”, which, in turn, is a continuation of Ser.No. 12/617,215, filed on Nov. 12, 2009 entitled “POINT-TO-MULTIPOINTDIGITAL RADIO FREQUENCY TRANSPORT” (which issued as U.S. Pat. No.8,326,218), which, in turn, is a continuation of U.S. application Ser.No. 10/740,944, filed on Dec. 19, 2003 entitled “POINT-TO-MULTIPOINTDIGITAL RADIO FREQUENCY TRANSPORT” (which issued as U.S. Pat. No.7,639,982), which, in turn, is a continuation of U.S. application Ser.No. 09/619,431, filed on Jul. 19, 2000, entitled “POINT-TO-MULTIPOINTDIGITAL RADIO FREQUENCY TRANSPORT” (which issued as U.S. Pat. No.6,704,545). All of the preceding applications and patents areincorporated herein by reference.

TECHNICAL FIELD

The present invention is related to high capacity mobile communicationssystems, and more particularly to a point-to-multipoint digitalmicro-cellular communication system.

BACKGROUND INFORMATION

With the widespread use of wireless technologies additional signalcoverage is needed in urban as well as suburban areas. One obstacle toproviding full coverage in these areas is steel frame buildings. Insidethese tall shiny buildings (TSBs), signals transmitted from wirelessbase stations attenuate dramatically and thus significantly impact theability to communicate with wireless telephones located in thebuildings. In some buildings, very low power ceiling mountedtransmitters are mounted in hallways and conference rooms within thebuilding to distribute signals throughout the building. Signals aretypically fed from a single point and then split in order to feed thesignals to different points in the building.

In order to provide coverage a single radio frequency (RF) source needsto simultaneously feeds multiple antenna units, each providing coverageto a different part of a building for example. Simultaneousbi-directional RF distribution often involves splitting signals in theforward path (toward the antennas) and combining signals in the reversepath (from the antennas). Currently this can be performed directly at RFfrequencies using passive splitters and combiners to feed a coaxialcable distribution network. In passive RF distribution systems, signalsplitting in the forward path is significantly limited due to inherentinsertion loss associated with the passive devices. Each split reducesthe level of the signal distributed in the building thereby makingreception, e.g. by cell phones, more difficult. In addition, the highinsertion loss of coaxial cable at RF frequencies severely limits themaximum distance over which RF signals can be distributed. Further, thesystem lacks any means to compensate for variations of insertion loss ineach path.

Another solution to distributing RF signals in TSBs is taking the RFsignal from a booster or base station, down converting it to a lowerfrequency, and distributing it via Cat 5 (LAN) or coaxial cable wiringto remote antenna units. At the remote antenna units, the signal is upconverted and transmitted. While down-conversion reduces insertion loss,the signals are still susceptible to noise and limited dynamic range.Also, each path in the distribution network requires individual gainadjustment to compensate for the insertion loss in that path.

In another approach, fiber optic cables are used to distribute signalsto antennas inside of a building. In this approach, RF signals arereceived from a bi-directional amplifier or base station. The RF signalsdirectly modulate an optical signal, which is transported throughout thebuilding as analog modulated light signals over fiber optic cable.Unfortunately, conventional systems using analog optical modulationtransmission over optical fibers require highly sophisticated linearlasers to achieve adequate performance. Also, analog optical systems arelimited in the distance signals can be transmitted in the building.Typically, this limitation is made worse due to the use of multimodefiber that is conventionally available in buildings. Multimode fiber iswider than single mode fiber and supports a number of differentreflection modes so that signals tend to exhibit dispersion at theterminating end of the fiber. In addition, analog installation typicallyincludes significant balancing when setting up the system. Further, RFlevels in the system need to be balanced with the optical levels. Ifthere is optical attenuation, the RF levels need to be readjusted. Inaddition, if the connectors are not well cleaned or properly secured,the RF levels can change.

Digitization of the RF spectrum prior to transport solves many of theseproblems. The level and dynamic range of digitally transported RFremains unaffected over a wide range of path loss. This allows for muchgreater distances to be covered, and eliminates the path losscompensation problem. However, this has been strictly a point-to-pointarchitecture. One drawback with digitally transported RF in apoint-to-point architecture is the equipment and cost requirement. Ahost RF to digital interface device is needed for each remote antennaunit. In particular, for use within a building or building complex thenumber of RF to digital interface devices and the fiber to connect thesedevices is burdensome. For example, in a building having 20 floors, therequirement may include 20 host RF to digital interface devices for 20remote antenna units, 1 per floor. In some applications more than oneremote antenna unit per floor may be required. As a result, there is aneed in the art for improved techniques for distributing RF signals inTSBs, which would incorporate the benefits of digital RF transport intoa point-to-multipoint architecture.

SUMMARY

The above-mentioned problems with distributing RF signals within abuilding and other problems are addressed by the present invention andwill be understood by reading and studying the following specification.

In one embodiment, a digital radio frequency transport system isprovided. The transport system includes a digital host unit and at leasttwo digital remote units coupled to the digital host unit. The digitalhost unit includes shared circuitry that performs bi-directionalsimultaneous digital radio frequency distribution between the digitalhost unit and the at least two digital remote units.

In another embodiment, a digital radio frequency transport system isprovided. The transport system includes a digital host unit and at leastone digital expansion unit coupled to the digital host unit. Thetransport system further includes at least two digital remote units,each coupled to one of the digital host unit and the digital expansionunit. The digital host unit includes shared circuitry that performsbi-directional simultaneous digital radio frequency distribution betweenthe digital host unit and the at least two digital remote units.

In an alternate embodiment, a method of performing point-to-multipointradio frequency transport is provided. The method includes receivingradio frequency signals at a digital host unit and converting the radiofrequency signals to a digitized radio frequency spectrum. The methodalso includes optically transmitting the digitized radio frequencyspectrum to a plurality of digital remote units. The method furtherincludes receiving the digitized radio frequency spectrum at theplurality of digital remote units, converting the digitized radiofrequency spectrum to analog radio frequency signals and transmittingthe analog radio frequency signals via a main radio frequency antenna ateach of the plurality of digital remote units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of one embodiment of a point-to-multipointcommunication system according to the teachings of the presentinvention.

FIG. 2 is a block diagram of one embodiment of a communication systemaccording to the teachings of the present invention.

FIG. 3 is a block diagram of another embodiment of a communicationsystem according to the teachings of the present invention.

FIG. 4 is a block diagram of one embodiment of a digital host unitaccording to the teachings of the present invention.

FIG. 5 is a block diagram of one embodiment of a digital remote unitaccording to the teachings of the present invention.

FIG. 6 is a block diagram of one embodiment of a digital expansion unitaccording to the teachings of the present invention.

FIG. 7 is a block diagram of one embodiment of a microcell base stationaccording to the teachings of the present invention.

FIG. 8 is an illustration of one embodiment of an overflow algorithm fora channel summer according to the teachings of the present invention.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific embodiments inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

FIG. 1 is an illustration of one exemplary embodiment of apoint-to-multipoint digital transport system shown generally at 100 andconstructed according to the teachings of the present invention. Thepoint-to-multipoint digital transport system 100 is shown distributedwithin a complex of tall shiny buildings (TSBs) 2. Although system 100is shown in a complex of TSBs 2, it is understood that system 100 is notlimited to this embodiment. Rather, system 100 in other embodiments isused to distribute signals in a single building, or other appropriatestructure or indoor or outdoor location that exhibits high attenuationto RF signals. Advantageously, system 100 uses digital summing ofdigitized RF signals from multiple antennas to improve signal coveragein structures, such as TSBs.

Point-to-multipoint digital transport of RF signals is accomplishedthrough a network of remote antenna units or digital remote units 40 and40′ and a digital host unit 20, which interfaces with a wireless network5 which is coupled to the public switched telephone network (PSTN), or amobile telecommunications switching office (MTSO) or other switchingoffice/network. System 100 operates by transporting RF signals digitallyover fiber optic cables. Signals received at DHU 20 are distributed tomultiple DRUs 40 and 40′ to provide coverage throughout a buildingcomplex. In addition, signals received at each of the DRUs 40 and 40′are summed together at the DHU 20 for interface to a wireless network.

In one embodiment, digital expansion unit DEU 30 is situated between theDHU 20 and one or more DRUs. In the forward path, DEU 30 expands thecoverage area by splitting signals received from DHU 20 to a pluralityof DRUs 40′. In the reverse path, DEU 30 receives signals from aplurality of DRUs 40′, digitally sums the signals together andtransports them to a DHU 20 or another DEU such as 30. This systemallows for successive branching of signals using DEUs 30 and expandedcoverage to multiple DRUs 40 and 40′. This system provides an efficientway of providing signal coverage for wireless communication withoutadded attenuation loss and distance constraint found with analogsystems. By using DEUs 30, antennas can be placed further from DHU 20without adversely affecting signal strength since shorter fiber opticcables can be used.

Digital transport system 100 includes a wireless interface device (WID)10 that provides an interface to a wireless network. In one embodiment,the WID 10 includes either conventional transmitters and receivers orall digital transmitter and receiver equipment, and interface circuitryto a mobile telecommunications switching office (MTSO). In oneembodiment, the wireless interface device 10 is coupled to an MTSO via aT1 line and receives and transmits signals between the MTSO and the DHU20. In another embodiment, the wireless interface device 10 is coupledto the public switched telephone network (PSTN). In one embodiment, WID10 comprises a base station and connects directly to DHU 20 via coaxialcables. In another embodiment, WID 10 comprises a base station andwirelessly connects to DHU 20 via a bi-directional amplifier that isconnected to an antenna. In one embodiment, the antenna is an outdoorantenna.

WID 10 communicates signals between wireless units and the wirelessnetwork via digital remote units DRUs 40 and 40′. WID 10 is coupled toDHU 20. The DHU 20 is coupled to at least one digital expansion unit DEU30 and a plurality of DRUs 40. In addition, DEU 30 is coupled to aplurality of DRUs 40′. The DHU 20 receives RF signals from WID 10 andconverts the RF signals to digital RF signals. DHU 20 further opticallytransmits the digital RF signals to multiple DRUs 40 either directly orvia one or more DEUs 30.

Each DRU 40 and 40′ is connected through a fiber optic cable (oroptionally another high bandwidth carrier) to transport digital RFsignals to one of DHU 20 or DEU 30. In one embodiment, the fiber opticcable comprises multimode fiber pairs coupled between the DRUs 40 andthe DHU 20, between the DRUs 40 and 40′ and the DEUs 30 and between theDEUs 30 and the DHU 20. In one embodiment, the DEU 30 is coupled to theDHU 20 via single mode fiber and the DEU 30 is coupled to the DRUs 40′via multimode fiber pairs. Although, transport system 100 has beendescribed with fiber optic cable other carriers may be used, e.g.,coaxial cable.

In another embodiment, the DHU 20 is coupled to the DRUs 40 by a directcurrent power cable in order to provide power to each DRU 40. In oneembodiment, the direct current power cable delivers 48 VDC to each DRU40 connected to the DHU 20. In another embodiment, the DEU 30 is coupledto DRUs 40′ by a direct current power cable to provide power to each DRU40′. In one embodiment, the direct current power cable delivers 48 VDCto each DRU 40′ connected to the DEU 30. In an alternate embodiment,DRUs 40 and 40′ are connected directly to a power supply. In oneembodiment, the power supply provides DC power to the DRUs 40 and 40′.In an alternate embodiment, the power supply provides AC power to theDRUs 40 and 40′.

In one embodiment, DRUs 40 and 40′ each include an AC/DC powerconverter. Both DHU 20 and DEU 30 split signals in the forward path andsum signals in the reverse path. In order to accurately sum the digitalsignals together at DHU 20 or DEU 30 the data needs to come in to theDHU 20 or DEU 30 at exactly the same rate. As a result all of the DRUs40 and 40′ need to be synchronized so that their digital sample ratesare all locked together. Synchronizing the signals in time isaccomplished by locking everything to the bit rate over the fiber. Inone embodiment, the DHU 20 sends out a digital bit stream and theoptical receiver at the DEU 30 or DRU 40 detects that bit stream andlocks its clock to that bit stream. In one embodiment, this is beingaccomplished with a multiplexer chip set and local oscillators, as willbe described below. Splitting and combining the signals in a digitalstate avoids the combining and splitting losses experienced with ananalog system. In addition, transporting the digital signals overmultimode fiber results in a low cost transport system that is notsubject to much degradation.

The down-conversion and up-conversion of RF signals are implemented bymixing the signal with a local oscillator (LO) at both the DRUs and theDHU. In order for the original frequency of the RF signal to berestored, the signal must be up-converted with an LO that has exactlythe same frequency as the LO that was used for down conversion. Anydifference in LO frequencies will translate to an equivalent end-to-endfrequency offset. In the embodiments described, the down conversion andup conversion LOs are at locations remote from one another. Therefore,in one preferred embodiment, frequency coherence between the local andremote LO's is established as follows: at the DHU end, there is a 142MHz reference oscillator which establishes the bit rate of 1.42 GHz overthe fiber. This reference oscillator also generates a 17.75 MHzreference clock which serves as a reference to which LO's at the DHU arelocked.

At each of the DRUs, there is another 17.75 MHz clock, which isrecovered from the optical bit stream with the help of the clock and bitrecovery circuits. Because this clock is recovered from the bit streamgenerated at the host, it is frequency coherent with the referenceoscillator at the host. A reference 17.75 MHz clock is then generated toserve as a reference for the remote local oscillators. Because theremote recovered bit clock is frequency coherent with the host masterclock, the host and remote reference clocks, and any LO's locked tothem, are also frequency coherent, thus ensuring that DHU and DRU LO'sare locked in frequency. It is understood that in other embodiments thebit rate over the fiber may vary and the frequency of the clocks willalso vary.

FIG. 2 is a block diagram of one embodiment of a communication system,shown generally at 200 and constructed according to the teachings of thepresent invention. In this embodiment, a digital host unit (DHU) 220 iscoupled to a bi-directional amplifier (BDA) 211. The BDA 211 receivescommunication signals from a wireless interface device (WID) andtransports the communication signals as RF signals to the DHU 220 andreceives RF signals from DHU 220 and transmits the RF signals to theWID. The DHU 220 receives RF signals from the BDA 211 and digitizes theRF signals and optically transmits the digital RF signals to multipleDRUs via transmission lines 214-1 to 214-N. DHU 220 also receivesdigitized RF signals over transmission lines 216-1 to 216-N from aplurality of DRUs either directly or indirectly via DEUs, reconstructsthe corresponding analog RF signals, and applies them to BDA 211. In oneembodiment, DHU 220 receives signals directly from a plurality N ofDRUs. The signals are digitally summed and then converted to analogsignals and transmitted to BDA 211. In another embodiment, DHU 220receives signals from one or more DEUs and one or more DRUs directly.Again, the signals are all digitally summed and then converted to analogsignals and transmitted to BDA 211. The signals received viatransmission lines 216-1 to 216-N may be received directly from a DRU orsignals that are received by a DEU and summed together and thentransported via 216-1 to 216-N to DHU 220 for additional summing andconversion for transport to BDA 211. DEUs provide a way to expand thecoverage area and digitally sum signals received from DRUs or other DEUsfor transmission in the reverse path to other DEUs or DHU 220. In oneembodiment, transmission lines 214-1 to 214-N and 216-1 to 216-Ncomprise multimode fiber pairs. In an alternate embodiment, each fiberpair is replaced by a single fiber, carrying bi-directional opticalsignals through the use of wavelength division multiplexing (WDM). In analternate embodiment, transmission lines 214-1 to 214-N and 216-1 to216-N comprise single mode fibers. In one embodiment, N is equal to six.In an alternate embodiment, the number of transmission lines in theforward path direction 214-1 to 214-N is not equal to the number oftransmission lines in the reverse path direction 216-1 to 216-N.

FIG. 3 is a block diagram of an alternate embodiment of a communicationsystem shown generally at 300 and constructed according to the teachingsof the present invention. Communication system 300 includes a basestation 310 coupled to a DHU 320. Base station 310 includes conventionaltransmitters and receivers 323 and 328, respectively, and conventionalradio controller or interface circuitry 322 to an MTSO or telephoneswitched network. DHU 320 is coupled to base station 310. DHU 320 isalso coupled to transmission lines 314-1 to 314-M, which transmit in theforward path direction and transmission lines 316-1 to 316-M, whichtransmit in the reverse path direction.

DHU 320 essentially converts the RF spectrum to digital in the forwardpath and from digital to analog in the reverse path. In the forwardpath, DHU 320 receives the combined RF signal from transmitters 323,digitizes the combined signal and transmits it in digital format overfibers 314-1 to 314-M, which are connected directly to a plurality ofDRUs or indirectly to one or more DRUs via one or more DEUs.

In one embodiment, DHU 320 receives signals directly from a plurality Mof DRUs. The signals are digitally summed and then converted to analogsignals and transmitted to base station 310. In another embodiment, DHU320 receives signals from one or more DEUs and one or more DRUsdirectly. Again, the signals are all digitally summed and then convertedto analog signals and transmitted to base station 310. The signalsreceived via transmission lines 316-1 to 316-M may be received directlyfrom a DRU or signals that are received by a DEU and summed together andthen transported via 316-1 to 316-M to DHU 320 for additional summingand conversion for transport to base station 210. DEUs provide a way toexpand the coverage area by splitting signals in the forward path anddigitally summing signals received from DRUs or other DEUs in thereverse path for transmission upstream to other DEUs or a DHU. In thereverse path, DHU 320 also receives digitized RF signals over fibers316-1 to 316-M from a plurality of DRUs, either directly or indirectlyvia DEUs, reconstructs the corresponding analog RF signal, and appliesit to receivers 328.

In one embodiment, transmission lines 314-1 to 314-M and 316-1 to 316-Mcomprise multimode fiber pairs. In an alternate embodiment, each fiberpair is replaced by a single fiber, carrying bi-directional opticalsignals through the use of wavelength division multiplexing (WDM). In analternate embodiment, transmission lines 314-1 to 314-M and 316-1 to316-M comprise single mode fibers. In one embodiment, M is equal to six.In an alternate embodiment, the number of transmission lines in theforward path direction 314-1 to 314-M is not equal to the number oftransmission lines in the reverse path direction 316-1 to 316-M.

Referring now to FIG. 4, there is shown one embodiment of a DHU 420constructed according to the teachings of the present invention. DHU 420includes an RF to digital converter 491 receiving the combined RFsignals from a wireless interface device such as a base station, BDA orthe like. RF to digital converter 491 provides a digitized trafficstream that is transmitted to multiplexer 466. Multiplexer 466 convertsthe parallel output of the A/D converter into a framed serial bitstream. At the output of the multiplexer is a 1 to P fan out buffer 407,which splits the digital signal P ways. There are P optical transmitters431-1 to 431-P one feeding each of the P optical transmission lines414-1 to 414-P. The digitized signals are applied to fibers 414-1 to414-P for transmission to corresponding DRUs either directly or viaDEUs. In one embodiment, P is equal to 6.

In one embodiment, DHU 420 includes an amplifier 450 that receives thecombined RF signal from a wireless interface device such as a basestation or BDA. The combined RF signal is amplified and then mixed bymixer 452 with a signal received from local oscillator 468. Localoscillator 468 is coupled to reference oscillator 415. In one embodimentthe local oscillator is coupled to a frequency divider circuit 470,which is in turn coupled to reference oscillator 415. The localoscillator is locked to the reference oscillator 415 as a master clockso that the down conversion of the RF signals is the same as the upconversion. The result is end to end, from DHU to DRU, or DHU to one ormore DEUs to DRU, no frequency shift in the signals received andtransmitted. The local oscillator 463 is also coupled to a synthesizercircuit 476.

The output signal of mixer 452 is provided to amplifier 454 amplifiedand then filtered via intermediate frequency (IF) filter 456. Theresultant signal is the combined RF signal converted down to an IFsignal. The IF signal is mixed with another signal originating from thereference oscillator 415 via mixer 460. The output of mixer 460 issummed together at 462 with a signal produced by field programmable gatearray (FPGA) 467. The output is then converted from an analog signal toa digital signal via analog/digital (A/D) converter 464 once convertedthe digital RF signal is applied to multiplexer 466. In one embodiment,the A/D converter 464 is a 14-bit converter handling a 14-bit signal. Inother embodiments, the A/D converter 464 may be of any size toaccommodate an appropriate signal. In one embodiment, the input signalfrom FPGA 467 is a dither signal from dither circuit 462 that addslimited out of band noise to improve the dynamic range of the RF signal.

In one embodiment, DHU 420 includes an alternating current to digitalcurrent power distribution circuit 6 that provides direct current powerto each of the DRUs coupled to DHU 420.

DHU 420 further includes a plurality of digital optical receivers 418-1to 418-P in the reverse path. Receivers 418-1 to 418-P each output anelectronic digital signal, which is applied to clock and bit recoverycircuits 445-1 to 445-P, respectively, for clock and bit recovery of theelectronic signals. The signals are then applied to demultiplexers 441-1to 441-P, respectively, which extract the digitized signals generated atthe DRUs, as will be explained in detail below. Demultiplexers 441-1 to441-P further extract alarm (monitoring) and voice information framedwith the digitized signals. The digitized signals output at eachdemultiplexer 441-1 to 441-P are then applied to FPGA 467 where thesignals are summed together and then applied to digital to RF converter495. Converter 495 operates on the sum of the digitized signalsextracted by demultiplexers 441-1 to 441-P, reconstructing basebandreplicas of the RF signals received at all the digital remote units. Thebaseband replicas are then up-converted to their original radiofrequency by mixing with a local oscillator 482 and filtering to removeimage frequencies. Local oscillator 482 is coupled to synthesizer 476and reference oscillator as discussed with respect to local oscillator468 above.

In one embodiment, digital to RF converter 495 includes digital toanalog (D/A) converter 484 coupled to an output of FPGA 467 thedigitized RF signals are converted to analog RF signals and then mixedwith a signal from reference oscillator 415 by mixer 492. The signal isthen filtered by IF filter 490 and amplified by amplifier 488. Theresultant signal is then mixed with a signal from local oscillator 482and then applied to RF filter 484, amplifier 480 and RF filter 478 fortransmission by a wireless interface device such as a BDA or basestation.

In one embodiment, FPGA 467 includes an alarm/control circuit 474, whichextracts overhead bits from DRUs to monitor error and alarm information.In one embodiment, the FPGA 467 includes a summer 498, whichmathematically sums together the digital RF signals received from fibers416-1 to 416-P. In another embodiment FPGA includes an overflowalgorithm circuit 486 coupled to the output of summer 486. The algorithmcircuit 496 allows the summed digital RF signals to saturate and keepthe summed signal within a defined number of bits. In one embodiment,the algorithm circuit includes a limiter. In one embodiment, the RFsignals are 14-bit signals and when summed and limited by summer 498 andoverflow algorithm 496 result in a 14-bit output signal.

For example, in one embodiment each of the digital RF signals receivedfrom fibers 416-1 to 416-P, where P is equal to 6, comprise 14 bitinputs. All of those 6 different 14 bit inputs then go into summer 498.In order to allow for overflow, at least 17 bits of resolution is neededin the summer 498 to handle a worst-case scenario when all 6 of the 14bit inputs are at full scale at the same time. In this embodiment, a17-bit wide summer 498 is employed to handle that dynamic range. Comingout of summer 498 is needed a 14-bit signal going in the reverse path.In one embodiment, an algorithm circuit 496 for managing the overflow isimplemented. In one embodiment, the summer and 498 and overflowalgorithm 496 are included in FPGA 467. In one embodiment, overflowalgorithm 496 acts like a limiter and allows the sum to saturate andkeeps the summed signal within 14 bits. In an alternate embodiment,overflow algorithm circuit 496 controls the gain and scales the signaldynamically to handle overflow conditions.

FIG. 8 illustrates one embodiment of an algorithm 863 for a channelsummer 865 in order to limit the sum of input signals 0 to 5 to 14 bits.In this embodiment, input signals 0 to 5 comprise 6 signals that aresummed together by summer 865. The sum of input signals 0 to 5 isreduced to a signal having 14 bits or less by the algorithm 863. It isunderstood that the algorithm 865 is by example and is not meant torestrict the type of algorithm used to limit the sum of signals 0 to 5to 14 bits or less.

For example, when the sum of the 6 input signals 0 to 5 is greater thanor equal to 13FFBh then the sum is divided by 6 for a signal that is 14bits or less. When the sum of the 6 input signals 0 to 5 is greater than13FFBh but less than or equal to FFFCh then the sum is divided by 5 fora signal that is 14 bits or less. When the sum of the 6 input signals 0to 5 is greater than FFFCh but less than BFFDh then the sum is dividedby 4 for a signal that is 14 bits or less. When the sum of the 6 inputsignals 0 to 5 is greater BFFDh but less than 7FFEh then the sum isdivided by 3 for a signal that is 14 bits or less. Finally, when the sumof the 6 input signals 0 to 5 is greater than 7FFEh but less than orequal to 3FFFh then the sum is divided by 2 for a signal that is 14 bitsor less.

FIG. 5 is a block diagram of one embodiment of a digital remote unit(DRU) 540 constructed according to the teachings of the presentinvention. A digital optical receiver 501 receives the optical digitaldata stream transmitted from a DHU directly or via a DEU. Receiver 501converts the optical data stream to a corresponding series of electricalpulses. The electrical pulses are applied to clock and bit recoverycircuit 503. The series of electrical pulses are then applied todemultiplexer 505. Demultiplexer 505 extracts the digitized trafficsignals and converts the signals from serial to parallel. The outputparallel signal is then applied to digital to RF converter 595 forconversion to RF and transmission to duplexer 547. RF converter 595 isconnected to the main antenna 599 through a duplexer 547. Accordingly,radio frequency signals originating from a wireless interface device aretransmitted from main antenna 547.

In one embodiment, digital to RF converter 595 includes adigital-to-analog (D/A) converter 509, which reconstructs the analog RFsignal and applies it to IF 504 and amplifier 506. The analog signal ismixed with an output signal of reference oscillator 515 by mixer 502.The output of amplifier 506 is mixed with a signal from local oscillator519 that locks the RF signal with the return digital signal viareference oscillator 515 that is coupled to local oscillator 519. In oneembodiment, the reference oscillator is coupled to frequency divider 517that in turn is coupled to local oscillators 519 and 529. The localoscillators 519 and 529 are also coupled to synthesizer 521 that iscoupled to programmable logic device 525.

RF signals received at main antenna 599 are passed through duplexer 547to RF to digital converter 593. The RF signals are converted to digitalsignals and then applied to multiplexer 536 converted fromparallel-to-serial and optically transmitted via optical transmitter 532to a DEU or DHU.

In one embodiment, RF to digital converter 593 includes a firstamplifier 543 that receives RF signals from duplexer 547, amplifies thesignals and transmits them to digital attenuator 539. In one embodiment,amplifier 543 is a low noise amplifier. Digital attenuator 539 receivesthe amplified signals and digitally attenuates the signal to control thelevels in case of an overload situation. RF to digital converter 593further includes a second amplifier 537, which receives the attenuatedsignals, amplifies the signals and applies the amplified signals tomixer 535. Mixer 535 mixes the amplified signals with a signal receivedfrom local oscillator 529. The resultant signals are applied to a thirdamplifier 533 an IF filter 548 and a fourth amplifier 546 in series todown convert to an IF signal. The IF signal is then mixed with a signalfrom reference oscillator 515 and the mixed signal is summed with asignal from dither circuit 527. The resultant signal is applied toanalog-to-digital converter 538 and converted to a digital signal. Theoutput digital signal is then applied to a multiplexer 536. In oneembodiment, the multiplexer 536 multiplexes the signal together with acouple of extra bits to do framing and control information. In oneembodiment, multiplexer 536, clock and bit recovery circuit anddemultiplexer 505 comprise a multiplexer chip set.

Programmable logic circuit 525 programs synthesizer 521 for thereference oscillator and for the up and down conversion of localoscillators 519 and 529. The programmable logic circuit 525 looks forerror conditions, for out of lock conditions on the oscillators andreports error modes and looks for overflow condition in the A/Dconverter 538. If an overflow condition occurs the programmable logiccircuit 525 indicates that you are saturating and adds some extraattenuation at digital attenuator 539 in order to reduce the RF signallevels coming in from RF antenna 599 and protect the system fromoverload.

In one embodiment, DRU 540 includes an internal direct current powerdistribution system 5. In one embodiment, the distribution systemreceives 48 VDC and internally distributes 3 outputs of +3.8V, +5.5V and+8V.

FIG. 6 is a block diagram of one embodiment of a digital expansion unit(DEU) 630 constructed according to the teachings of the presentinvention. DEU 630 is designed to receive optical signals and transmitoptical signals. An optical receiver 651 receives digitized RF signalsand transmits them to clock and bit recovery circuit 653 that performsclock and bit recovery to lock the local clock and clean up the signal.The signals are then split into X RF digital signals by 1 to X fan outbuffer 607. The signals are then transmitted via optical transmitters655-1 to 655-X to X receiving units such as DEUs or DRUs. The Xreceiving units may be any combination of DEUs or DRUs. In oneembodiment, X is equal to six.

DEU 630 also includes optical receivers 669-1 to 669-X, which receivedigitized RF signals directly from DRUs or indirectly via DEUs. Inoperation the signals are received, applied to clock and bit recoverycircuits 673-1 to 673-X respectively to lock the local clock and cleanup the signals and then applied to demultiplexers 671-1 to 671-X.Demultiplexers 671-1 to 671-X each extract the digitized traffic andapply the samples to field programmable gate array 661. The signals aresummed together digitally and transmitted to multiplexer 657, whichmulitplexes the signal together with a couple of extra bits to doframing and control information. In addition, the multiplexer 657converts the signals parallel to serial. The signals are then applied tooptical transmitter 659 for further transmission. In one embodiment, thesignals are directly transmitted to a DHU or indirectly via one or moreadditional DEUs.

In one embodiment, the FPGA 661 includes summer 665, whichmathematically sums together the digital RF signals received fromdemultiplexers 671-1 to 671-X. In another embodiment, FPGA 661 includesan overflow algorithm circuit 663 coupled to the output of summer 665.The algorithm circuit 663 allows the summed digital RF signals tosaturate and keep the summed signal within a defined number of bits. Inone embodiment, the algorithm circuit includes a limiter. In oneembodiment, the RF signals are 14-bit signals and when summed andlimited by summer 665 and overflow algorithm 663 result in a 14-bitoutput signal.

In one embodiment, DEU 630 includes an alternating current to digitalcurrent power distribution circuit 7 that provides direct current powerto each of the DRUs coupled to DEU 630.

In an alternate embodiment, the digital host unit (DHU) and wirelessinterface device (WID) are located some distance from the building beingserved. The DHU in the building is replaced by a DEU, and the linkbetween that DEU and the remotely located DHU is via single mode fiber.FIG. 7 is a block diagram of this embodiment. A microcell base stationshown generally at 700 includes conventional transmitters and receivers723 and 728, respectively, and conventional radio controller orinterface circuitry 722. In the forward path, a DHU 767 receives thecombined RF signal from transmitters 723, digitizes the combined signaland transmits it in digital format over single mode fiber to a DEU. Inthe reverse path, DHU 767 receives digitized RF signal from a DEU,reconstructs the corresponding analog RF signal, and applies it toreceivers 728.

In another alternate embodiment, the wireless interface device (WID) isa software defined base station, and the interface between the DHU andWID takes place digitally, eliminating the need for the RF to digitalconversion circuitry in the DHU.

CONCLUSION

A digital radio frequency transport system has been described. Thetransport system includes a digital host unit and at least two digitalremote units coupled to the digital host unit. The digital host unitincludes shared circuitry that performs bi-directional simultaneousdigital radio frequency distribution between the digital host unit andthe at least two digital remote units.

In addition, a digital radio frequency transport system has beendescribed. The transport system includes a digital host unit and atleast one digital expansion unit coupled to the digital host unit. Thetransport system further includes at least two digital remote units,each coupled to one of the digital host unit and the digital expansionunit. The digital host unit includes shared circuitry that performsbi-directional simultaneous digital radio frequency distribution betweenthe digital host unit and that at least two digital remote units.

Further, a method of performing point-to-multipoint radio frequencytransport has been described. The method includes receiving analog radiofrequency signals at a digital host unit and converting the analog radiofrequency signals to digitized radio frequency signals. The method alsoincludes splitting the digitized radio frequency signals into aplurality of a digital radio frequency signals and opticallytransmitting the digital radio frequency signals to a plurality ofdigital remote units. The method further includes receiving the digitalradio frequency signals at a plurality of digital remote units,converting the digital radio frequency signals to analog radio frequencysignals and transmitting the signals via a main radio frequency antennaat each of the plurality of digital remote units.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. For example, adigital remote unit is not limited to the receipt and summing andsplitting and transmitting of digitized radio frequency signals. Inother embodiments, the digital host unit is capable of receiving andsumming analog radio frequency signals in addition to or instead ofdigitized radio frequency signals. As well, the digital host unit iscapable of splitting and transmitting analog radio frequency signals inaddition to or instead of digitized radio frequency signals. Thisapplication is intended to cover any adaptations or variations of thepresent invention. Therefore, it is intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. A system for digital transport of a wirelessspectrum, the system comprising: a first unit; and a plurality of secondunits communicatively coupled to the first unit using at least one wiredcommunication link and remotely located from the first unit; whereineach of the plurality of second units receives a respective analogwireless signal comprising the wireless spectrum and any transmissionsfrom any wireless units within a respective coverage area associatedwith that second unit; wherein each of the plurality of second units isconfigured to generate respective digital samples indicative of at leasta portion of the wireless spectrum of the respective analog wirelesssignal received at that second unit; wherein each of the plurality ofsecond units is configured to communicate the respective digital samplesgenerated by that second unit to the first unit using the at least onewired communication link; wherein the first unit is configured todigitally sum corresponding digital samples derived from the digitalsamples received from the plurality of second units to produce summeddigital samples; and wherein the system is configured so that an inputused for base station processing is derived from the resulting summeddigital samples.
 2. The system of claim 1, wherein at least some of theplurality of second units are communicatively coupled to the first unitat least in part using a separate wired communication link.
 3. Thesystem of claim 1, wherein at least some of the plurality of secondunits are communicatively coupled to the first unit at least in partusing a shared wired communication link.
 4. The system of claim 1,wherein at least some of the plurality of second units arecommunicatively coupled to the first unit using an intermediary digitalexpansion unit.
 5. The system of claim 1, wherein the system includesbase station signal processing functionality configured to perform basestation processing.
 6. The system of claim 5, wherein the base stationprocessing functionality is at least one of: co-located with the firstunit and remote from the first unit.
 7. The system of claim 1, whereinthe system comprises at least a portion of a base station for performingthe base station processing.
 8. The system of claim 1, wherein thesystem further comprises software configured to perform the base stationprocessing using the input derived from the resulting summed digitalsamples.
 9. The system of claim 8, wherein the first unit comprises thesoftware configured to perform the base station processing using theinput derived from the resulting summed digital samples.
 10. The systemof claim 1, wherein the system is configured to convert the summeddigital samples to a replicated analog wireless signal.
 11. The systemof claim 10, wherein the system comprises a digital-to-analog converterto convert the summed digital samples to the replicated analog wirelesssignal.
 12. The system of claim 1, wherein each of the plurality ofsecond units comprises: a respective analog-to-digital converter todigitize the respective analog wireless signal received at that secondunit in order to produce the respective digital samples.
 13. The systemof claim 1, wherein each of the plurality of second units generates therespective digital samples indicative of the respective analog wirelesssignal received at that second unit by frequency shifting the respectiveanalog wireless signal received at that second unit and digitizing thefrequency-shifted analog wireless signal.
 14. The system of claim 1,wherein each of the plurality of second units is configured to generatethe respective digital samples indicative of the respective analogwireless signal received at that second unit by down converting therespective analog wireless signal received at that second unit anddigitizing the down-converted analog wireless signal.
 15. The system ofclaim 1, wherein the first unit is configured to digitally sum thecorresponding digital samples derived from the digital samples receivedfrom the plurality of second units as a part of generating the summeddigital samples at a resolution greater than a resolution of the digitalsamples being summed.
 16. The system of claim 1, wherein the wirelessspectrum comprises a cellular radio frequency spectrum and any modulatedtransmissions from the wireless units carried by the cellular radiofrequency spectrum of the wireless spectrum, and wherein the digitalsamples generated by and received from each of the second units comprisedigital radio frequency samples in that the digital samples areindicative of the modulation of the transmissions carried by thecellular radio frequency spectrum of the wireless spectrum.
 17. Thesystem of claim 16, wherein each of the second units is configured toperform a down-conversion in connection with generating the digitalradio frequency samples.
 18. The system of claim 17, wherein each of thesecond units is configured to perform a down-conversion of an analogsignal in connection with generating the digital radio frequencysamples.
 19. The system of claim 1, wherein the at least one wiredcommunication link comprises at least one of: a single mode opticalfiber, a multi-mode optical fiber, a coaxial cable, and a categorycable.
 20. A first unit used in digital transport of a wirelessspectrum, the first unit comprising: at least one interface tocommunicatively couple the first unit to a plurality of second unitsusing at least one wired communication link, wherein each of theplurality of second units is remotely located from the first unit andeach of the plurality of second units receives a respective analogwireless signal comprising the wireless spectrum and any transmissionsfrom any wireless units that are within a coverage area associated withthat second unit and wherein each of the plurality of second units isconfigured to generate respective digital samples indicative of at leasta portion of the wireless spectrum of the respective analog wirelesssignal received at that second unit, wherein the interface is used toreceive the digital samples from the plurality of second units; adigital summer to digitally sum corresponding digital samples derivedfrom the digital samples received from the plurality of second units toproduce summed digital samples; and wherein the first unit is configuredso that an input used for base station processing is derived from theresulting summed digital samples.
 21. The first unit of claim 20,wherein at least some of the plurality of second units arecommunicatively coupled to the first unit at least in part using aseparate wired communication link, wherein the first unit comprises aseparate interface configured to communicatively couple the first unitto each of said at least some of the plurality of second units using therespective separate wired communication link.
 22. The first unit ofclaim 20, wherein at least some of the plurality of second units arecommunicatively coupled to the first unit at least in part using ashared wired communication link.
 23. The first unit of claim 20, whereinat least some of the plurality of second units are communicativelycoupled to the first unit using an intermediary digital expansion unit.24. The first unit of claim 20, wherein the base station processing isat least one of: co-located with the first unit and remote from thefirst unit.
 25. The first unit of claim 20, wherein the base stationprocessing is performed by at least a portion of a base station.
 26. Thefirst unit of claim 20, wherein the first unit further comprisessoftware configured to perform the base station processing using theinput derived from the resulting summed digital samples.
 27. The firstunit of claim 20, wherein the first unit is configured to convert thesummed digital samples to a replicated analog wireless signal.
 28. Thefirst unit of claim 27, wherein the first unit comprises adigital-to-analog converter to convert the summed digital samples to thereplicated analog wireless signal.
 29. The first unit of claim 20,wherein the first unit is configured to digitally sum the correspondingdigital samples derived from the digital samples received from theplurality of second units in order to produce the summed digital samplesat a resolution greater than a resolution of the digital samples beingsummed.
 30. The first unit of claim 20, wherein the wireless spectrumcomprises a cellular radio frequency spectrum.
 31. A method for digitaltransport of a wireless spectrum used with a first unit and a pluralityof second units, each of the plurality of second units being remotelylocated from the first unit, the method comprising: at each of theplurality of second units: receiving a respective analog wireless signalcomprising the wireless spectrum and any transmissions from any of thewireless units that are within a coverage area associated with thatsecond unit; generating respective digital samples indicative of atleast a portion of the wireless spectrum of the respective analogwireless signal received at that second unit; and communicating therespective digital samples generated by that second unit to the firstunit using at least one communication medium; and at the first unit,digitally summing corresponding digital samples derived from the digitalsamples received from the plurality of second units to produce summeddigital samples.
 32. The method of claim 31, wherein at least some ofthe plurality of second units are communicatively coupled to the firstunit at least in part using a separate wired communication link.
 33. Themethod of claim 31, wherein at least some of the plurality of secondunits are communicatively coupled to the first unit at least in partusing a shared wired communication link.
 34. The method of claim 31,wherein at least some of the plurality of second units arecommunicatively coupled to the first unit using an intermediary digitalexpansion unit.
 35. The method of claim 31, further comprisingperforming base station processing using an input derived from theresulting summed digital samples.
 36. The method of claim 35, whereinthe base station processing is at least one of: co-located with thefirst unit and remote from the first unit.
 37. The method of claim 35,wherein the base station processing is performed by at least a portionof a base station.
 38. The method of claim 31, further comprisingconverting the summed digital samples to a replicated analog wirelesssignal.
 39. The method of claim 31, wherein the corresponding digitalsamples derived from the digital samples received from the plurality ofsecond units are digitally summed at a resolution greater than aresolution of the digital samples being summed.
 40. The method of claim31, wherein the wireless spectrum comprises a cellular radio frequencyspectrum.
 41. The method of claim 31, wherein, at each of the pluralityof second units, generating the respective digital samples indicative ofthe at least a portion of the wireless spectrum of the respective analogwireless signal received at that second unit comprises: frequencyshifting the respective analog wireless signal received at that secondunit; and digitizing the frequency-shifted analog wireless signal. 42.The method of claim 31, wherein, at each of the plurality of secondunits, generating the respective digital samples indicative of the atleast a portion of the wireless spectrum of the respective analogwireless signal received at that second unit comprises: down convertingthe respective analog wireless signal received at that second unit; anddigitizing the down-converted analog wireless signal.
 43. A system fordigital transport of a wireless spectrum, the system comprising: a firstunit; and a plurality of second units communicatively coupled to thefirst unit using at least one wired communication link, wherein each ofthe plurality of second units is remotely located from the first unit;wherein each of the plurality of second units is configured to receive arespective analog wireless signal comprising the wireless spectrum;wherein each of the plurality of second units is configured to generaterespective digital samples indicative of at least a portion of thewireless spectrum of the respective analog wireless signal received atthat second unit; wherein each of the plurality of second units isconfigured to communicate the respective digital samples generated bythat second unit to the first unit using the at least one wiredcommunication link; wherein the first unit is configured to digitallysum corresponding digital samples derived from the digital samplesreceived from the plurality of second units to produce summed digitalsamples; and wherein the system is configured so that an input used forwireless protocol processing is derived from the resulting summeddigital samples.
 44. The system of claim 43, wherein at least some ofthe plurality of second units are communicatively coupled to the firstunit at least in part using a separate wired communication link.
 45. Thesystem of claim 43, wherein at least some of the plurality of secondunits are communicatively coupled to the first unit at least in partusing a shared wired communication link.
 46. The system of claim 43,wherein at least some of the plurality of second units arecommunicatively coupled to the first unit using an intermediary digitalexpansion unit.
 47. The system of claim 43, wherein the wirelessprotocol processing is at least one of: co-located with the first unitand remote from the first unit.
 48. The system of claim 43, wherein thesystem comprises at least a portion of a base station for performing thewireless protocol processing.
 49. The system of claim 43, wherein thesystem further comprises software configured to perform the wirelessprotocol processing using the input derived from the resulting summeddigital samples.
 50. The system of claim 49, wherein the first unitcomprises the software configured to perform the wireless protocolprocessing using the input derived from the resulting summed digitalsamples.
 51. A first unit for digital transport of a wireless spectrum,the first unit comprising: at least one interface to communicativelycouple the first unit to a plurality of second units using at least onewired communication link, wherein each of the plurality of second unitsis remotely located from the first unit and each of the plurality ofsecond units is configured to receive a respective analog wirelesssignal comprising the wireless spectrum and wherein each of theplurality of second units is configured to generate respective digitalsamples indicative of at least a portion of the wireless spectrum of therespective analog wireless signal received at that second unit, whereinthe at least one interface is configured to receive the digital samplesfrom the plurality of second units at the first unit; a digital summerto digitally sum corresponding digital samples derived from the digitalsamples received from the plurality of second units to produce summeddigital samples; and wherein the first unit is configured so that aninput used for wireless protocol processing is derived from theresulting summed digital samples.
 52. The first unit of claim 51,wherein at least some of the plurality of second units arecommunicatively coupled to the first unit at least in part using aseparate wired communication link, wherein the first unit comprises aseparate interface configured to communicatively couple the first unitto each of said at least some of the plurality of second units using therespective separate wired communication link.
 53. The first unit ofclaim 51, wherein at least some of the plurality of second units arecommunicatively coupled to the first unit at least in part using ashared wired communication link.
 54. The first unit of claim 51, whereinat least some of the plurality of second units are communicativelycoupled to the first unit using an intermediary digital expansion unit.55. The first unit of claim 51, wherein the wireless protocol processingis at least one of: co-located with the first unit and remote from thefirst unit.
 56. The first unit of claim 51, wherein the wirelessprotocol processing is performed by at least a portion of a basestation.
 57. The first unit of claim 51, wherein the first unit furthercomprises software configured to perform the wireless protocolprocessing using the input derived from the resulting summed digitalsamples.